Geology Reference
In-Depth Information
or
CA
¼
K
0
1
K
CO
2
pCO
2
H
fg
þ
2 K
0
1
K
0
2
K
CO
2
pCO
2
ð
4
:
12
Þ
H
f
2
This equation can be rearranged to give the following quadratic
expression that can be solved for the pH
CA H
f
2
K
0
1
K
CO
2
pCO
2
H
fg
2K
0
1
K
0
2
K
CO
2
pCO
2
¼
0
Worked Example 1
For seawater (35 psu, 15 1C) with an alkalinity of 2.30 meq l
1
and in
equilibrium with atmospheric CO
2
¼
3.65
10
4
atm, calculate (i)
the pH and (ii) the speciation of carbonic acid.
(i) pH calculation
Data, including constants from Table 4 are K
CO2
¼
10
1.41
, K
0
1
¼
10
6.05
, K
0
2
¼
10
9.23
,CA
¼
2.30
10
3
, pCO
2
¼
3.65
10
4
The pH is obtained from the calculation of {H
1
} using the above
quadratic equation. Thus
H
fg¼
b
þ
p
b
2
4ac
2a
where
a
¼
CA
¼
2
:
30
10
3
b
¼
K
1
K
CO2
pCO
2
¼
10
6
:
5
10
1
:
41
10
3
:
46
¼
1
:
20
10
11
c
¼
2K
1
K
2
K
CO2
pCO
2
¼
210
6
:
05
10
9
:
23
10
1
:
41
10
3
:
46
¼
1
:
42
10
20
Giving {H
1
}
¼
6.22
10
9
and pH
¼
8.21.
(ii) Carbonic acid speciation calculations
Knowing the pH, each of the three major species (H
2
CO
3
, HCO
3
,
CO
3
2
) can be calculated as a fraction (or percentage) of the
P
CO
2
.
